1. Field of the Invention
The present invention relates to a multilevel AC/DC power converting method and converter device thereof. More particularly, the present invention relates to the multilevel AC/DC power converting method and converter device thereof for enhancing the efficiency of power conversion.
2. Description of the Related Art
In general, a conventional AC/DC power converter circuit includes a diode rectifier, a thyristor phase-controlled rectifier or the like. However, an input AC current of the diode rectifier or the thyristor phase-controlled rectifier contains serious harmonic distortion and a relatively low input power factor. Disadvantageously, the harmonic distortion generated from the diode rectifier or the thyristor phase-controlled rectifier fails to meet the requirements of a harmonic control standard.
Furthermore, the conventional AC/DC power converter circuit also includes a high-frequency power converter which is formed with a structure of bridge-type power electronic switches. The power electronic switches of the high-frequency power converter are controlled by high-frequency PWM (Pulse Width Modulation) to adjust an input current approaching to a sinusoidal waveform and a phase of the input current same with that of a voltage of the AC power source, such that it can approach to a unity of the input power factor. However, in switching the power electronic switches of the high-frequency power converter with high frequency, a higher switching power loss occurs due to a higher voltage change in each switching operation. Disadvantageously, it results in a lower efficiency of power conversion.
For example, U.S. PATENT PUBLICATION No. 20080031014 entitled “AC/DC Converter Comprising Plural Converters” and U.S. PATENT PUBLICATION No. 20050156579 entitled “Multiphase Converter With Zero Voltage Switching” disclose various power converters. The above-mentioned patents and publications are incorporated herein by reference for purposes including, but not limited to, indicating the background of the present invention and illustrating the state of the art.
In order to reduce the switching power loss in high-frequency switching, the voltage change in each switching operation of the power electronic switch must be reduced. Hence, a multilevel AC/DC power converter device is developed. A structural circuit of the multilevel AC/DC power converter device mainly includes a diode-clamping type, a flywheel capacitor type and a cascade bridge type. The voltage change in each switching operation of the power electronic switch of the multilevel AC/DC converter device is reduced and generates a multilevel AC voltage at an input terminal. Consequently, the multilevel AC/DC power converter device reduces the switching power loss, increases the efficiency of power conversion, and reduces filtering inductances and degrees of electromagnetic interferences.
The cascade bridge type requires the least number of power electronic switches if the three types of the above-mentioned multilevel AC/DC power converter device are designed to generate the same levels of AC input voltage. However, each power converter of the conventional cascade bridge-type AC/DC power converter device must be controlled by high-frequency PWM which results in a complication of the driving circuit and a higher degree of switching power loss. Furthermore, the conventional cascade bridge-type AC/DC power converter device has a critically technical limit, since each power converter must output a DC power. Unavoidably, the conventional cascade bridge-type AC/DC power converter device must have several separate DC outputs formed with no common connection point. Accordingly, an additional power conversion circuit must be applied to integrate all of the separate DC outputs into a single DC output which further results in a complication of the power circuit and a higher degree of switching power loss.
As is described in greater detail below, the present invention provides a multilevel AC/DC power converting method and converter device thereof. A cascade structure of the multilevel AC/DC power converter device is formed from a high-frequency power converter and a low-frequency power converter serially connected. A DC voltage of the low-frequency power converter is relatively high and operated by low-frequency switching, while a DC voltage of the high-frequency power converter is relatively low and operated by high-frequency switching. In comparison with the conventional cascade structure, it can reduce the switching loss and can increase the efficiency of power conversion. Due to only outputting a DC power at the low-frequency power converter, no real power will be input or output at the high-frequency power converter, and, thus, no additional power conversion circuit will be provided for integrating all separate DC outputs into a single DC output. Advantageously, the entire structure is further simplified, and the manufacturing cost is further reduced in such a way to mitigate and overcome the above problem.